Read-out device and method for reading out X-rays stored in phosphor layers

ABSTRACT

In order to improve the picture quality of X-rays, a read-out control is provided for controlling a read-out device such that a first storage phosphor layer, which has a first thickness d 1,  is read out by the read-out device controlled in a first read-out mode, and a second storage phosphor layer, which has a second thickness d 2  which is greater than the first thickness d 1,  is read out by the read-out device controlled in a second read-out mode, the second read-out mode being different from the first read-out mode. The scan parameters of the read-out device set in the first read-out mode are different from the scan parameters set in the second read-out mode in at least one scan parameter. The scan parameters are, for example, the size of the pixels, a pulse duration and/or intensity and/or width of a focus range of stimulation light, integration time and/or feed time of the detector.

The invention relates generally to a read-out device and to acorresponding method for reading out X-rays stored in storage phosphorlayers.

BACKGROUND OF THE INVENTION

One possibility for recording X-ray pictures is storing the X-rayradiation passing through an object, for example a patient, as a latentpicture in a phosphor layer. In order to read out the latent picture,the phosphor layer is irradiated with stimulation light, and sostimulated into emitting emission light. The emission light, theintensity of which corresponds to the picture stored in the phosphorlayer, is collected by an optical detector and converted into electricalsignals. The electrical signals are further processed as required, andfinally made available for analysis, in particular formedical/diagnostic purposes, when they are displayed on an appropriatedisplay unit, such as a monitor or printer.

With generic read-out devices according to the prior art, the picturequality of the read out X-ray required or prescribed for a reliablemedical diagnosis is not achieved in all applications.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a read-out device and acorresponding method with which the highest possible picture quality ofthe read out X-ray is achieved.

In order to improve the picture quality of X-rays, a read-out control isprovided for controlling a read-out device such that a first storagephosphor layer, which has a first thickness d1, is read out by theread-out device controlled in a first read-out mode, and a secondstorage phosphor layer, which has a second thickness d2 which is greaterthan the first thickness d1, is read out by the read-out devicecontrolled in a second read-out mode, the second read-out mode beingdifferent from the first read-out mode. The scan parameters of theread-out device set in the first read-out mode are different from thescan parameters set in the second read-out mode in at least one scanparameter. The scan parameters are, for example, the size of the pixels,a pulse duration and/or intensity and/or width of a focus range ofstimulation light, integration time and/or feed time of the detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first variation of a read-out device;

FIG. 2 shows an example of a phosphor layer to be read out;

FIG. 3 shows a second variation of a read-out device; and

FIG. 4 shows an example of a radiography system.

DETAILED DESCRIPTION OF THE PREFERRRED EMBODIMENTS

According to the invention, a read-out control is provided whichcontrols the read-out device in such a way that a first storage phosphorlayer, which has a first thickness, is read out by the read-out devicecontrolled in a first read-out mode, and a second storage phosphorlayer, which has a second thickness which is greater than the firstthickness, is read out by the read-out device controlled in a secondread-out mode, the second read-out mode being different from the firstread-out mode.

The invention is based upon the idea of controlling the read-out devicedependent upon the thickness of the respective phosphor layer to be readout. This is achieved in that at least part of the scan parameters ofthe read-out device set when reading out the phosphor layer is setdependent upon the thickness of the respective phosphor layer to be readout. The scan parameters include preferably:

-   -   the size of the pixels of the X-ray picture read out,    -   the pulse duration and/or intensity of the stimulation light,    -   the width of the focus range of the stimulation light on the        phosphor layer,    -   the integration time and sampling rate of the detector, and    -   the feed time of the detector and the relative speed with which        the read-out device and the phosphor layer are moved relative to        one another during the read-out.

According to the invention, the scan parameters set in the firstread-out mode are different from the scan parameters set in the secondread-out mode, in at least one scan parameter. This means that at leastone scan parameter in the first read-out mode has another value than inthe second read-out mode.

By means of the control according to the invention of the scanparameters of the read-out device dependent upon the thickness of thephosphor layer to be read out, improved adaptation both of the read-outconditions and the thickness of the phosphor layer to the respectivemedical use, which is also called application, is made possible. In thisway, a respective optimal picture quality can be achieved for differentapplications. Different applications include but are not limited toX-rays of limbs, of the scull, of the spinal column, of the thorax, ofthe abdomen or of the pelvis.

In a first embodiment of the invention, when reading out a phosphorlayer, the read-out device can produce a picture made up of a largenumber of pixels, the size of the pixels produced being smaller in thefirst read-out mode than in the second read-out mode. In this way, theresolution of the picture obtained is increased, and the picture qualityis further improved.

The read-out device has an irradiation device for irradiating a phosphorlayer with stimulation light which can stimulate the phosphor layer intoemitting emission light. Moreover, the read-out device includes adetector for collecting emission light stimulated in the phosphor layer,it being possible for the detector and the phosphor layer to be movedrelative to one another.

Preferably, the irradiation device is controlled by the read-out controlsuch that the irradiation device emits stimulation pulses with aspecific pulse duration, the pulse duration of the stimulation lightpulses being shorter in the first read-out mode than in the secondread-out mode. In this way, with each read-out of a pixel, in particularof a line of pixels, a smaller partial region of the phosphor layer ispassed over with the stimulation light in the first read-out mode thanin the second read-out mode. The stimulated partial regions of thephosphor layer are correspondingly smaller. The size of the partialregions stimulated can thus be adapted to the required size of thepixels.

It is moreover advantageous to control the irradiation device by meansof the read-out control such that the intensity of the stimulation lightin the first read-out mode is less than the intensity of the stimulationlight in the second read-out mode. In this way, in the first read-outmode a diffusion of the stimulation light within the phosphor layer isreduced in comparison to the second read-out mode such that a higherpicture definition is achieved. This applies in particular to so-calledPowder Image Plates (PIP). With these PIP, the storage phosphorparticles, which are substantially isotropic in form, are mixed inpowder form with a binding agent and processed into a layer.

Preferably, a focussing device is provided for focussing the stimulationlight on the storage phosphor layer, the stimulation light hitting thestorage phosphor layer in a focus range, which is in particular linear,and the width of the focus range in the first read-out mode beingsmaller than in the second read-out mode. In this way, with eachread-out of individual pixels, in particular of a whole line of pixels,a smaller partial region of the storage phosphor layer is passed over bythe stimulation light in the first read-out mode than in the secondread-out mode. In this way too, the picture definition is increased.

In a further preferred example, the detector is controlled by theread-out control such that the emission light is collected in a largenumber of time intervals during the relative movement, the timeintervals being shorter in the first read-out mode than in the secondread-out mode.

In the case of line detector which has a large number of light-sensitiveelements arranged in a line, the time intervals are the so-calledintegration times within which the light-sensitive elements collectlight and convert it into corresponding electrical signals. In the caseof a non-locally resolving detector, such as a photomultiplier, the timeintervals are sampling times within which the analogue signal producedby the detector is sampled. The sampling time is the reciprocal value ofthe so-called sampling frequency which is also called the sampling rate.

Overall, in this way a high resolution with at the same time reducednoise portions and reduced movement blur, which is caused by therelative movement of the detector and the storage phosphor layer areachieved in the first read-out mode, and so the picture quality isfurther improved.

Preferably, the detector and the phosphor layer are moved relative toone another with a relative speed, the relative speed being lower in thefirst read-out mode than in the second read-out mode. In this way too,the movement blur is reduced, and at the same time the signal/noiseratio of the picture data obtained is improved.

It is preferred that the first thickness of the first phosphor layer isbetween 100 and 750 μm. With phosphor layers in this range ofthicknesses, the picture definition is only slightly effected by thediffusion of stimulation light in the layer, but at the same time asufficiently high intensity of the emission light emitted by thephosphor layer is achieved. This has a particularly positive effect uponthe picture quality with applications where picture definition is vital.

The second thickness of the second phosphor layer is preferably between750 and 1000 μm. With phosphor layers in this thickness range, inapplications in which a particularly high picture definition can bedispensed with, picture data with a particularly favourable signal/noiseratio and contrast can be obtained.

Preferably, the first and/or second phosphor layer used hasneedle-shaped storage phosphor structures. These phosphor layers arealso called Needle Image Plates (NIP) and supply picture data with anexceptional signal/noise ratio and high picture definition.

FIG. 1 shows a first variation of a read-out device, which in thefollowing example is also called a scanner. A phosphor layer 1 to beread out, which has a thickness d, is disposed on a support layer 2 andis irradiated with stimulation light 3 which is produced by a lightsource 4. The emission light 7 stimulated by the stimulation light 3 inthe phosphor layer 1 is collected by a detector 9. The light source 4and the detector 9, including an imaging device 8 and an optical filter11, together form the scanner 10 which is moved in the feed direction Vrelative to the phosphor layer 1 during the read-out.

The light source 4 has several individual radiation sources 5, such aslight diodes or laser diodes, and a focussing device 6 which focus thestimulation light bundles 12 coming from the radiation sources 5 ontothe phosphor layer 1.

The focussing device 6 has two oblong cylinder lenses which extendsubstantially parallel to the individual radiation sources 5 arranged ina line. The divergent stimulation light bundles 12 coming from theindividual radiation sources 5 are bundled by the focussing device 6 andstrike the phosphor layer 1 as a convergent radiation bundle of thestimulation light 3. The divergent stimulation light bundles 12 of theindividual radiation sources 5 overlap at right angles to the plane ofFIG. 1 in such a way that the convergent radiation bundle describes acontinuous stimulation light line 13 on the phosphor layer 1 extendingat a right angle to the plane of FIG. 1.

The emission light 7 stimulated and radiated in the region of thestimulation light line 13 in the phosphor layer 1 is collected by adetector 9 such as to be locally resolved. For this, the detector 9 hasa large number of light-sensitive detector elements 14 which arearranged along a line extending at right angles to the plane of FIG. 1.The emission light 7 emitted in the region of the stimulation light line13 on the phosphor layer 1 is reproduced on the light-sensitive detectorelements 14 of the detector 9 by an imaging device 8. The detector 9 ispreferably in the form of a CCD or photodiode line.

Suitable as an imaging device 8 are preferably microlenses which arearranged along a line extending at right angles to the plane of FIG. 1.Alternatively, gradient index lenses are also suitable for this, inparticular self-focussing lenses which are also arranged in a lineextending at right angles to the level of the Figure. Preferably, theindividual regions 15 are reproduced by the imaging device 8 on thelight-sensitive surfaces 14 of the detector 9 on the scale of 1:1.

In the example shown, the scanner 10 is moved by a conveyance mechanism(not shown) in feed direction V over the latent phosphor layer 1,different linear regions 15 of the phosphor layer 1 being successivelystimulated by the stimulation light line 13, and the respectivelyemitted emission light 7 being collected, locally resolved, by thelight-sensitive detector elements 14 of the detector 9.

Preferably, the light source 4 is disposed in front of the detector 9 inrelation to feed direction V, i.e. the scanner 10 runs with the lightsource 4 over the phosphor layer 1. In this way it is possible for alarger portion of the stimulation light 3 diffused in the phosphor layerto be diffused in the direction of regions 15 which have already beenread out, whereas only a smaller portion of the stimulation light 3 isdiffused in the direction of the regions 15 of the phosphor layer 1which have not yet been read out. In this way, intensity and definitionlosses due to the diffusion of stimulation light 3 within the phosphorlayer 1 can be reduced.

In the example shown, the scanner 10 is moved over a fixed phosphorlayer 1. Of course, the above also applies when the scanner 10 is fixedand the phosphor layer 1 disposed on the support layer 2 is conveyed inmovement direction P relative to this. The same applies similarly toembodiments with which the scanner 10 is moved in feed direction V, andthe phosphor layer 1 is moved in movement direction P.

While the scanner 10 is moved in feed direction V relative to thephosphor layer 1, the stimulation light line 13 passes over individualregions 15 of the phosphor layer 1, and stimulates these, one after theother, into emitting emission light 7 which is collected by the detector9 for each of the individual regions 15.

For the movement by the width of a region 15, the detector 9 requires afeed time Tv. In the course of this movement, the light-sensitivesurfaces 14 of the detector 15 collect the emission light 7 emitted fromthe region 15 during an integration time T₁.

The width of the regions 15 in feed direction V is typically betweenapproximately 10 μm and 500 μm. The crossways extension of thelight-sensitive surfaces 14 of the detector 9 at right angles to thedirection of the detector line is typically between approximately 10 μmand 600 μm.

Preferably, the crossways extension of the light-sensitive surfaces 14is greater than the width of the regions 15 in feed direction V. Forexample, the width of the regions 15 is approximately 50 μm, whereas thecrossways extension of the light-sensitive surfaces 14 is approximately400 μm. The width of a single region 15 in feed direction V is in thiscase given by the width of the section of the phosphor layer 1 which thestimulation light line 13 stimulates into emitting emission light 7while it is being fed in feed direction V within the feed time T_(v).The local resolution in feed direction V, i.e. the smallest possiblewidth of an individual region 15 is determined here by the width of thefocus range of the stimulation light line 13 in feed direction V.

For better clarity, the regions 15 of the phosphor layer 1 and thelight-sensitive surfaces 14 of the detector 9 in FIG. 1 are all greatlyenlarged, and not shown truly to scale.

Advantageously, the integration time T₁ is shorter than the feed timeT_(v), i.e. T₁<T_(v). In this way, the thermally generated dark noise isreduced with respect to methods known from the prior art, and so overallthe signal/noise ratio is improved. Because the detector 9 only passesover part of the width of the region 15 when collecting emission light 7during the integration time T₁, information losses which can be causedby so-called movement blur, are furthermore reduced.

FIG. 2 shows a top view of an example of a phosphor layer 1 to be readout. In the representation chosen here, the individual regions 15 of thephosphor layer 1 are also shown greatly enlarged—as in FIG. 1—forreasons relating to better clarity.

On the phosphor layer 1, a stimulation light line 13 is drawn and movedwith the light source 4 and the detector 9, including the imaging device8 and the filter 11 (see FIG. 1 ) in feed direction V relative to thestorage phosphor layer 1, and thus passes over the individual regions15. For feeding by the width of a region 15, the detector 9 or thestimulation light line 13 requires a specific feed time T_(v) which, inthe case of a constant relative speed, corresponds to the quotient ofthe width B_(v) of the individual regions 15 and the relative speed.

The light source 4 is controlled in such a way that only a respectivefirst partial region 16 of the regions 15 is irradiated directly withstimulation light, whereas a second partial region 17 of the regions 15is not irradiated directly with stimulation light. The light source 4here only emits stimulation light when the stimulation light line 13passes over the first partial region 16. The light source 4 is thenswitched off so that a from now on “virtual stimulation light line”passes over the second partial region 17 without irradiating this withstimulation light. Correspondingly, only the first partial region 16 isstimulated into emitting emission light by direct irradiation withstimulation light. The second partial region 17, on the other hand, isstimulated into emitting emission light by the stimulation light hittingthe first partial region 16 and partially diffused in the phosphor layer1 in feed direction V.

The light source 4 is controlled here by specifying a stimulation timeT_(s) during which the light source 4 is switched on with the movementof the stimulation light line 13 over a region 15. After the stimulationtime T_(s) is over, the light source 4 remains switched off until thestimulation light line 13 has reached a region 15 which is the next tobe read out, upon which the irradiation cycle described can start onceagain by turning on the light source 4. The aperture of the detector 9which depends upon the extension of the light-sensitive surfaces 14 ofthe detector 9 and the imaging device 8 thus also passes over theindividually stimulated regions 15 of the phosphor layer 1, one afterthe other. The light-sensitive surfaces 14 of the detector 9 arranged ina line thus collect the emission light 7 respectively emitted by thelinear regions 15.

The detector 9 is thus controlled in a way such that it only collectsemission light 7 emitted from the region 15 during an integration timeT₁. The integration time T₁ is shorter here than the feed time T_(v)which the detector 9 and the light source 4 require in order to cover adistance corresponding to the width B_(v) of the regions 15 in feeddirection V. The integration time T₁ is preferably synchronised with thestimulation time T_(s), i.e. these begin at the same time and are of thesame duration.

With respect to methods known from the prior art, with which theintegration time is identical to the feed time, the shortening of theintegration time with respect to the feed time described here leads toconsiderably reduced dark noise. Because the aperture of the detector 9consequently passes over a narrower section of the region 15 in feeddirection V while the emission light is collected during the shorterintegration time, at the same time the movement blur and so a resultingloss of information during read-out is reduced.

As can also be seen in FIG. 2, the individual regions 15 are eachsub-divided into a large number of individual pixels 18. Thissub-division is achieved by collecting the emission light emitted fromthe linear regions 15 with a linearly formed detector 9, the width B_(z)of the pixels 18 corresponding to the extension of the individuallight-sensitive detector elements 14 in the line direction of thedetector 9. The width Bz of the pixels 18 is typically betweenapproximately 10 μm and 500 μm, preferably approximately 50 μm.

The emission light 7 emitted by the individual pixels 18 and collectedline by line by the corresponding light-sensitive surfaces 14 of thedetector 9 is converted into corresponding detector signals in thedetector 9 which represent the picture information of the latent X-raypicture read out.

The read-out of the phosphor layer 1 is preferably controlled by a pulsesignal. The pulse signal has a periodic progression of individualrectangular pulses, the pulse duration of which corresponds to thestimulation time T_(s) and the integration time T₁. The distance in timebetween the ascending sides of two subsequent pulses corresponds here tothe feed time T_(v). With the periodical pulse signal of this example,the feed time T_(v) thus corresponds to the period duration of the pulsesignal.

Preferably, as already explained, the stimulation time T_(s) and theintegration T₁ are identical, i.e. the stimulation and collection of theemission light happen synchronously. Alternatively, however, it is alsopossible to control the read-out process with two different pulsesignals which differ from one another in the duration of the pulses(i.e. the stimulation time T_(s) is then different from the integrationtime T₁) and/or the phase position of the pulses relative to one another(i.e. the stimulation time T_(s) begins at a different point in timethan does the integration time T₁).

With this variation of the scanner 10, at least part of the scanparameters of the scanner 10 during read-out is chosen dependent uponthe thickness d of the phosphor layer 1 to be read out or upon aread-out control (not shown). Alternatively or in addition, at leastpart of the scan parameters can be set dependent upon the body part, theX-ray of which is stored in the phosphor layer to be read out.

Scan parameters here are preferably to be understood as being one ormore of the following parameters, already described in greater detailabove:

-   -   the size, i.e. the width B_(z) or B_(v) of the pixels 18 of the        X-ray picture read out;    -   the stimulation time T_(s) and the pulse duration of the        stimulation light pulses;    -   the intensity of the stimulation light 3;    -   the width of the focus range 13 of the stimulation light 3 on        the phosphor layer 1;    -   the integration time T₁ of the detector 9;    -   the feed time T_(v) of the detector 9 and the relative speed        with which the scanner 10 and the phosphor layer 1 are moved        relative to one another during the read-out.

The read-out of a phosphor layer 1 with a first thickness d1 occurs withdifferent scan parameters than the read-out of a phosphor layer 1 with asecond thickness d2 which is greater than the first thickness d1.

Alternatively or in addition, the read-out of a phosphor layer 1, inwhich an X-ray of a first body part such as a limb or a scull is stored,occurs with different scan parameters than the read-out of a phosphorlayer 1 in which an X-ray of a second body part, such as a spinalcolumn, a thorax, an abdomen or a pelvis, is stored.

FIG. 3 shows a second variation of a read-out device for reading out aphosphor layer 1. A laser 32 produces a stimulation light beam 33 whichis deflected by a deflection element 34 set in rotation by a motor 35such that this moves along a line 38 over the phosphor layer 1 to beread out. The deflection element 34 is preferably in the form of amirror, in particular a polygon mirror or a galvanometer mirror.

During the movement of the stimulation light beam 33 along the line 38,the phosphor layer 1 emits emission light dependent upon the X-rayinformation stored therein, which is gathered by an optical gatheringdevice 36, for example a light conductor bundle, conveyed on andcollected by an optical detector 37, preferably a photomultiplier,coupled to the gathering device 36, and converted into a correspondinganalogue detector signal S.

The detector signal S is fed into a processing unit 46 in which picturesignal values S_(B) are deduced for individual pixels of the X-raypicture read out. By conveying the phosphor layer 1 in conveyancedirection P, a successive read-out of individual lines 38 is achieved,and so a two-dimensional X-ray picture made up of individual pixels witha respectively associated picture signal value S_(B) is obtained. If thenumber of lines 38 read out in conveyance direction V is for example1500, for example with 1000 pixels respectively per line 38, a total of1500×1000 pixels each with an associated picture signal value S_(B) areobtained for the X-ray picture read out.

In the embodiment shown here, the analogue detector signal S is first ofall filtered through a low pass filter 42, higher frequency portions ofthe detector signal S, in particular noise portions, being eliminated orat least reduced. The filtered, analogue detector signal S is fed into adigitizing device 43, and there sampled and digitized with apre-specified sampling frequency, a digital detector signal value Dbeing obtained in digital units for each sampling process. The samplingfrequency, which is also called the sampling rate, is typically between1 and 12 MHz.

The sampling of the analogue detector signal S in the digitizing device43 preferably happens according to the so-called Sample and HoldPrinciple with which the respective current analogue signal height ofthe detector signal S given by the digitizing unit 43 at a sampling timeis held and converted into a corresponding digital detector signal valueD.

From the digital detector signal values D are intermediately stored in astorage unit 44, the individual picture signal values S_(B) are finallyestablished in a calculation unit 45, in that two or more detectorsignal values D are combined into one pixel and from this, for exampleby forming an average value, a picture signal value S_(B) belonging tothis pixel is calculated.

The sampling frequency is preferably chosen such that for eachindividual pixel along the line 38, at least two digital detector signalvalues D are obtained, from which a picture signal value S_(B)respectively belonging to a pixel can be calculated.

With this variation of the read-out device according to the inventiontoo, at least part of the scan parameters is chosen or set by a read-outcontrol (not shown) dependent upon the thickness d of the phosphor layer1 to be read out. Alternatively or in addition, at least part of thescan parameters can be set dependent upon the body part, the X-ray ofwhich is stored in the phosphor layer to be read out.

Scan parameters here are preferably to be understood as being one ormore of the following parameters, already described above:

-   -   the size of the pixels of the X-ray picture read out;    -   the intensity of the stimulation light 33;    -   the width of the focus range, i.e. of the line 38 of the        stimulation light 33 on the phosphor layer 1;    -   the sampling frequency or sampling rate when sampling the        analogue detector signal S; and    -   the relative speed with which the phosphor layer 1 is moved        relative to the line 38 of the stimulation light during the        read-out.

The read-out of a phosphor layer 1 with a first thickness d1 occurs withdifferent scan parameters than the read-out of a phosphor layer 1 with asecond thickness d2, which is greater than the first thickness d1.

Alternatively, or in addition, the read-out of a phosphor layer 1, inwhich an X-ray of a first body part, such as a limb or a scull isstored, takes place with different scan parameters than the read-out ofa phosphor layer 1, in which an X-ray of a second body part, such as aspinal column, a thorax, an abdomen or a pelvis, is stored.

FIG. 4 shows an example of a radiography system for recording X-rays onphosphor layers. In this example, a phosphor layer (as illustrated e.g.in FIGS. 1 to 3) is located in an X-ray cassette 19 which is inserted inan X-ray table 20. The X-ray table 20 includes an X-ray foot 23 in whichthe X-ray cassette 19 is located, and a supporting surface 24 positionedon the X-ray foot 23 on which, when taking X-rays, patients or theirlimbs can be laid. An X-ray radiation source 21 is disposed over thesupporting surface 24, and this can emit X-ray radiation 25 withdifferent energy and intensity in the direction of the supportingsurface 24.

The energy limit of the X-ray radiation used for the X-ray is chosen orcorrespondingly set by a recording control (not shown) dependent uponthe respective thickness d (see FIG. 1 or 3) of the phosphor layer used.With an X-ray using a phosphor layer with a first thickness d1, a firstenergy limit is set for the X-ray radiation. With an X-ray using aphosphor layer with a second thickness d2 which is greater than thefirst thickness d1, a second energy limit is set for the X-rayradiation, which is greater than the first energy limit.

The energy limit of the X-ray radiation is to be understood here asmeaning the maximum energy of the respective X-ray photons. The value ofthe respective energy limit of the X-ray photons, e.g. 50 keVcorresponds here to the value of the high voltage set in the X-ray tubesin which the X-ray radiation is produced, therefore 50 kV in theaforementioned example.

By controlling the radiography system dependent upon the thickness ofthe phosphor layer used in the recording, improved adaptation of therecording conditions and of the thickness of the phosphor layer to therespective medical application is achieved. In this way, a respectivelyoptimal picture quality can be achieved for different applications.Different applications include X-rays of limbs, the scull, the spinalcolumn, the thorax, the abdomen or the pelvis.

Preferably, the first energy limit of the X-ray radiation of theradiography system is in the range of between 40 and 70 keV. With theseenergies, in the first phosphor layer an X-ray picture is stored which,despite the lesser thickness of the phosphor layer, results in arelatively high intensity of the emission light, and also a highsignal/noise ratio during read-out.

The second energy limit of the X-ray radiation of the radiography systemis preferably in the range of between 70 and 150 eV. With theseenergies, in the second phosphor layer an X-ray picture is stored which,due to the greater thickness of the phosphor layer, results in anequally relatively high intensity of the emission light, and also a highsignal/noise ratio during read-out.

Preferably, during an X-ray, the intensity of the X-ray radiation usinga phosphor layer with a first thickness is greater than with an X-rayusing a phosphor layer with a second thickness which is greater than thefirst thickness. In this way, still better adaptation of the picturequality of the recorded X-ray required for the respective application isachieved.

1. A read-out device for reading out X-rays stored in phosphor layers,comprising: a read-out control for controlling the read-out device suchthat, a first storage phosphor layer, which has a first thickness d1, isread out from the read-out device controlled in a first read-out mode,and a second storage phosphor layer, which has a second thickness d2which is greater than the first thickness d1, is read out from theread-out device controlled in a second read-out mode, the secondread-out mode being different from the first read-out mode.
 2. Theread-out device according to claim 1, further comprising means toproduce a picture made up from a large number of pixels during theread-out of the storage phosphor layer, the size (B_(z), B_(v)) of thepixels being smaller in the first read-out mode than in the secondread-out mode.
 3. The read-out device according to claim 1, furthercomprising an irradiation device for irradiating a storage phosphorlayer with stimulation light which can stimulate the storage phosphorlayer into emitting emission light, and the irradiation device beingcontrolled by the read-out control such that the irradiation deviceemits stimulation light pulses with a specific pulse duration T_(s), thepulse duration T_(s) of the stimulation light pulses being shorter inthe first read-out mode than in the second read-out mode.
 4. Theread-out device according to claim 1, further comprising an irradiationdevice for irradiating a storage phosphor layer with stimulation lightwhich can stimulate the storage phosphor layer into emitting emissionlight, and the irradiation device being controlled by the read-outcontrol such that the intensity of the stimulation light in the firstread-out mode is less than the intensity of the stimulation light in thesecond read-out mode.
 5. The read-out device according to claim 1,further comprising an irradiation device for irradiating a storagephosphor layer with stimulation light which can stimulate the storagephosphor layer into emitting emission light, and a focussing device forfocussing the stimulation light onto the storage phosphor layer, thestimulation light hitting the storage phosphor layer in a focus regionwhich is in particular linear, and a width of the focus range beingsmaller in the first read-out mode than in the second read-out mode. 6.The read-out device according to claim 1, further comprising a detectorfor collecting emission light stimulated in the storage phosphor layer,the detector and the storage phosphor layer able to be moved relative toone another, and the detector being controlled by the read-out controlsuch that the emission light is collected in a large number of timeintervals T₁ during the relative movement, the time intervals T₁ in thefirst read-out mode being shorter than in the second read-out mode. 7.The read-out device according to claim 1, further comprising a detectorfor collecting emission light stimulated in the storage phosphor layer,the detector and the storage phosphor layer able to be moved relative toone another with a relative speed, the relative speed being smaller inthe first read-out mode than in the second read-out mode.
 8. Theread-out device according to claim 1, wherein the first thickness d1 ofthe first storage phosphor layer is between 100 and 750 μm.
 9. Theread-out device according to claim 1, wherein the second thickness d2 ofthe second storage phosphor layer is between 750 and 1000 μm.
 10. Theread-out device according to claim 1, the first and/or the secondstorage phosphor layer having needle-shaped storage phosphor structures.11. A method for reading out X-rays stored in storage phosphor layers,comprising the steps of: reading out first data from a first storagephosphor layer having a first thickness d1 by a read-out devicecontrolled in a first read-out mode; and reading out second data from asecond storage phosphor layer having a second thickness d2 greater thanthe first thickness d1 by the read-out device controlled in a secondread out mode, the second read-out mode being different from the firstread-out mode.